Gustatory sensory cells express a receptor responsive to protein breakdown products (GPR92)

Abstract

The ingestion of dietary protein is of vital importance for the maintenance of fundamental physiological processes. The taste modality umami, with its prototype stimulus, glutamate, is considered to signal the protein content of food. Umami was thought to be mediated by the heterodimeric amino acid receptor, T1R1 + T1R3. Based on knockout studies, additional umami receptors are likely to exist. In addition to amino acids, certain peptides can also elicit and enhance umami taste suggesting that protein breakdown products may contribute to umami taste. The recently deorphanized peptone receptor, GPR92 (also named GPR93; LPAR5), is expressed in gastric enteroendocrine cells where it responds to protein hydrolysates. Therefore, it was of immediate interest to investigate if the receptor GPR92 is expressed in gustatory sensory cells. Using immunohistochemical approaches we found that a large population of cells in murine taste buds was labeled with an GPR92 antibody. A molecular phenotyping of GPR92 cells revealed that the vast majority of GPR92-immunoreactive cells express PLCβ2 and can therefore be classified as type II cells. More detailed analyses have shown that GPR92 is expressed in the majority of T1R1-positive taste cells. These results indicate that umami cells may respond not only to amino acids but also to peptides in protein hydrolysates.

Fig. 1

RT-PCR analysis for the peptone receptor GPR92 and ribosomal protein l8 (RpL8) in murine taste papillae. Reverse transcription polymerase chain reaction (RT–PCR) experiments were performed with primer pairs specific for mGPR92 (492 bp) and a control housekeeping mRNA, mRpL8 (202 bp), respectively. Amplicons of the expected size could be observed in all analysed taste papillae types (circumvallate papillae CvP, foliate papillae FoP, fungiform papillae FuP). No bands were observed in negative controls lacking template (neg.)

Fig. 2

Visualization of GPR92 in murine taste papillae. Immunolabeling of cross sections through the murine circumvallate papillae (a–d), the foliate papillae (e–f) or the fungiform papillae (g). (a, b) Numerous GPR92-positive cells are located within taste buds of the circumvallate papilla. (c, d) Magnification of a circumvallate taste bud. GPR92-positive cells show the typical, spindle-shaped morphology of taste cells. (e) GPR92-immunoreactive cells in taste buds of foliate papillae. (f) Magnification of the dotted area in (e). GPR92-immunoreactive cells in foliate taste buds. (g) GPR92-positive cell in a fungiform papilla. Sections are counterstained with DAPI (blue). Scale bars: a, b, f = 20 μm; c, d, g = 10 μm; e = 50 μm

Fig. 3

Intrinsic GFP-fluorescence in the circumvallate papilla of a PLCβ2 transgenic mouse. (a) Overview of a cross section through the circumvallate papilla. GFP-positive cells (green) are located within taste buds. (b, c) Magnification of the dotted area in (a). GFP-positive cells show the typical, spindle-shaped morphology of taste cells. Sections are counterstained with DAPI (blue). Scale bars: a = 100 μm; b, c = 20 μm

Fig. 4

GPR92 is expressed in type II taste cells. Immunohistochemistry employing an GPR92 antibody on cross sections through the circumvallate papilla of a PLCβ2 transgenic mouse. (a) GPR92-immunoreactive cells in circumvallate taste buds. (b) Numerous GFP-positive cells in the same circumvallate taste buds. (c) The overlay of (a) and (b) clearly reveals that the vast majority of GPR92-immunoreactive cells is also GFP-positive. However, a subset of GFP-expressing cells shows no labeling for GPR92 (arrowhead). (d) Quantitative analyses of co-expression patterns. GPR92-positive cells, PLCβ2-GFP-positive cells and double-labeled cells (GPR92-positive and PLCβ2-GFP-positive) in the circumvallate papilla were counted on cross sections. The analyses revealed that 91.4% of GPR92-immunoreactive cells are GFP-positive. In contrast, 72.0% of GFP-expressing cells show GPR92-immunoreactivity. Twenty sections from the circumvallate papilla of one mouse were analyzed. Data are expressed as mean numbers ± SD. Sections are counterstained with DAPI (blue). Scale bars: a–c = 20 μm

Fig. 5

GPR92 is expressed in T1R1-expressing taste cells of the circumvallate papilla. Immunohistochemistry on cross sections through the circumvallate papilla of a T1R1-mCherry gene-targeted mouse. (a) The GPR92 antibody labels numerous cells in taste buds of the circumvallate papilla. (b) T1R1-expressing taste cells visualized by their intrinsic mCherry-fluorescence. (c) Overlay of (a) and (b); the vast majority of GPR92-immunoreactive cells shows also mCherry-fluorescence. (d, e, f) Magnification of the dotted area in (c). GPR92 is co-expressed in mCherry-labeled cells. (g) Quantitative analyses of co-expression patterns. GPR92-positive cells, mCherry-positive cells and double-labeled cells (GPR92-positive and T1R1-mCherry-positive) were counted on cross sections through the circumvallate papillae of T1R1-mCherry gene-targeted mice. About 94.5% of GPR92-labeled cells show mCherry-fluorescence and 97.9% of T1R1-mCherry expressing cells are GPR92-immunoreactive. Thirty-six sections from circumvallate papillae of two mice were analyzed. Data are expressed as mean numbers ± SD. Sections are counterstained with DAPI (blue). Scale bars: a–c = 20 μm; d–f = 10 μm